In the manufacturing of an electronic device such as a semiconductor device, a plasma processing apparatus is used for the processes of etching, film forming, and the like. A capacitively coupled plasma processing apparatus is known as a kind of a plasma processing apparatus. In general, the capacitively coupled plasma processing apparatus includes a chamber main body, an upper electrode, and a lower electrode. The upper electrode and the lower electrode are disposed such that a space in a chamber, which is defined by the chamber main body, is interposed therebetween. In such a plasma processing apparatus, a gas is supplied into the chamber, and a high-frequency electric field is generated between the upper electrode and lower electrode. The gas is excited by the high-frequency electric field to generate plasma. A workpiece is processed by means of ions and/or radicals from the plasma.
As the capacitively coupled plasma processing apparatus, a plasma processing apparatus that uses two high-frequency power sources and a plasma processing apparatus that uses a single high-frequency power source are known. In the former apparatus, i.e., the plasma processing apparatus that uses two high-frequency power sources, a first high-frequency power source is connected to the upper electrode or the lower electrode while a second high-frequency power source is connected to the lower electrode. The second high-frequency power source generates a high-frequency power having a frequency lower than that of a high-frequency power generated by the first high-frequency power source.
Meanwhile, a transformer is used in the latter apparatus, i.e., the plasma processing apparatus that uses a single high-frequency power source. A primary-side coil of the transformer is connected to the high-frequency power source. One end of a secondary-side coil is connected to an upper electrode, and the other end of the secondary-side coil is connected to a lower electrode. The secondary-side coil includes a plurality of tabs. In such a plasma processing apparatus, it is possible to select a ratio of a first high-frequency power output from one end of the secondary-side coil to a second high-frequency power output from the other end of the secondary-side coil by selecting a ground tab from the plurality of tabs.
In the related art, a division ratio of the secondary-side coil of a transformer is selected according to the selection of the tab. Accordingly, a selectable ratio of the first high-frequency power to the second high-frequency power, i.e., a selectable power ratio of two output high-frequency powers, is large and discrete. Thus, it is impossible to minutely control the power ratio of two output high-frequency powers. In addition, it is impossible to change the power ratio of two output high-frequency powers in a state in which a high-frequency power is supplied to the primary-side coil. Furthermore, it is impossible to switch a phase difference between the first high-frequency power and the second high-frequency power from in-phase to antiphase or vice versa.